The present invention relates to a solder paste suitable for use in mounting chip components on a printed circuit board by reflow soldering in a furnace.
In view of the recent trend toward reduction in size and weight of electronic equipment, electronic devices used in electronic equipment may have very small dimensions. For example, chip components called 1005, which are useful as passive devices such as chip capacitors and chip resistors, have rectangular dimensions of 1 mm by 0.5 mm, which could not be expected a decade ago.
As well known in the art, chip components are leadless electronic devices, particularly passive electronic devices, suitable for surface mount technology, like QFPs and SOICs in active devices (IC packages), by being mounted directly on a printed circuit board having metallic pads (lands) so as to bond the electrodes of the chip components to the lands of the circuit board through solder.
In general, soldering methods which can be applied to electronic devices include iron soldering, flow soldering, and reflow soldering.
Iron soldering is performed manually by an operator who holds a soldering iron and a flux-cored wire solder in his hands. This soldering method is not usable for soldering of chip components since a large number of chip components are densely mounted on a printed circuit board with tiny spaces between adjacent chip components, which are too small for a soldering iron to be inserted into the spaces to perform soldering. In addition, the productivity of iron soldering is unacceptably low because the large number of chip components mounted on the circuit board must be soldered one by one.
Flow soldering is carried out by contacting the surface of a printed circuit board with molten solder. In this soldering method, a large number of soldered joints can be formed in a single operation, resulting in good productivity.
However, since the printed circuit board is positioned with its surface to be soldered facing down, chip components directly mounted on the circuit board must be previously fixed thereon with an adhesive. Accordingly, flow soldering method requires an additional bothersome step of application of an adhesive to the areas of the printed circuit boards on which chip components are to be mounted.
Furthermore, in the case where a large number of chip components are densely mounted with small spaces on a circuit board, flow soldering may cause a soldering defect called bridging, which results from adhesion of the solder to the circuit board so as to connect adjacent chip components. The occurrence of bridging on a printed circuit board is entirely unacceptable since it results in malfunction of the electronic equipment containing the components. Moreover, since the flow soldering method causes the entire lower surface of the printed circuit board to contact the molten solder, the solder may adhere to undesirable areas on that surface, leading to failure due to an unexpected electrical connection.
Reflow soldering is typically performed by using a solder paste, which is a highly viscous paste formed by uniformly admixing a fine powder solder with a flux, which is a fluid typically comprising a rosin or other organic resin, an activator, a solvent, and optionally a thixotropic agent.
The solder paste is applied by screen printing through a metal mask or silk screen to the areas on the surface of a printed circuit board on which chip components or other electronic devices are to be mounted. The electronic devices are then mounted on the applied solder paste and temporarily fixed in place by the tackiness or adhesion of the applied solder paste. The printed circuit board having the electronic devices mounted thereon is then heated, usually in a furnace, to melt the solder powder in the paste, and it is then cooled to solidify the molten solder, thereby forming soldered joints between the mounted electronic devices and the circuit board.
In the reflow soldering method, since the solder paste can be applied solely to the desired areas on the printed circuit board by printing, adhesion of the solder to undesirable areas on the circuit board can be avoided and the occurrence of bridging is minimized, resulting in an improvement in reliability of soldering. Furthermore, its productivity is good since there is no need to apply an adhesive before electronic devices are mounted on the circuit board and a large number of soldered joints can be formed by a single operation of the printing and heating steps. In view of these advantages, most chip components and other electronic devices are currently soldered by the reflow soldering method.
The powder solders used in conventional solder pastes for reflow soldering are mostly formed from a eutectic solder alloy which has an alloy composition equal or close to the eutectic Sn--Pb alloy (61.9% Sn--39.1% Pb), e.g., a composition consisting of about 63% Sn and about 37% Pb. The eutectic solder alloys have a eutectic temperature (at which the solidus temperature is the same as the liquidus temperature) at about 183.degree. C. which is the lowest melting temperature of Sn--Pb based solder alloys.
In general, a soldering temperature (soldering peak temperature) of about 50.degree. C. higher than the liquidus temperature of the solder alloy is considered to be most suitable. In reflow soldering, the entire printed circuit board is heated in a furnace to cause the applied solder paste to reflow, and therefore the electronic devices mounted on the circuit board are inevitably exposed to the same temperature as the solder paste. In order to minimize the effect of heat on the electronic devices in the heating step, the use of a solder alloy having a liquidus temperature as low as possible is recommended so as to decrease the soldering temperature. For this reason, the above-described eutectic solder alloys have been mostly used in the prior art in reflow soldering.
When a eutectic solder alloy is used to solder chip components by reflow soldering, however, tombstoning (also called the Manhattan phenomenon), which is a phenomenon in which a chip component is detached from the printed circuit board at one end while remaining bonded to the circuit board at the opposite end, whereby the one end rises and the chip component assumes a more or less vertical orientation, takes place frequently. Tombstoning is caused by a difference of time at which the solder paste is melted at opposite ends during heating in a furnace. Namely, when melting of the solder paste which adheres to a first electrode located at one end of the chip component occurs earlier than melting of the solder paste adhering to a second electrode located at the opposite end of the chip component, the earlier melted solder paste acts to pull down the first electrode by the surface tension force of the molten solder. As a result, the opposite end of the chip component on which the second electrode is located is detached from the circuit board and raised toward an upright position, since the solder paste adhering to the second electrode has not been melted and no pulling force is exerted on the second electrode.
The tombstoning phenomenon can be prevented by controlling the phases appearing during transformation of the solder alloy from a liquid phase to a solid phase. Japanese Patent Application Laid-Open (Kokai) No. 5-212580(1993) proposes that the use of a "twin-peak" solder alloy, which has two peaks in a DSC (differential scanning calorimeter) curve, in reflow soldering of chip components is effective for preventing the tombstoning phenomenon. The twin-peak solder alloys disclosed in that patent application include such alloys as Sn--3%Ag--1%Cu--4%Pb, Sn--10%Zn--4%Pb, Sn--5%Pb--1.2%Ag, and Sn--10%Pb--1.2%Ag, in which all the percents are by weight.
The DSC curve of a eutectic Sn--Pb solder alloy (63%Sn--Pb) is shown in FIG. 2. As can be seen from this figure, only a single peak appears in the DSC curve and the range between the solidus and liquidus temperatures, i.e., solidification temperature range, is relatively narrow.
Although the above-described twin-peak solder alloys have a significant effect on prevention of tombstoning, they have a higher liquidus temperature which exceeds 200.degree. C. Therefore, the soldering temperature, which is the actual temperature at which soldering is performed, should be 250.degree. C. or higher, which may cause thermal damage to the chip components or other electronic devices mounted on the circuit board. In addition, these twin-peak alloys have another disadvantage in that their soldering properties are slightly inferior to those of the eutectic solder alloys.